route Command

Introduction

The route command is a traditional Linux networking utility used to view and manipulate the IP routing table. It allows system administrators to configure static routes, set up default gateways, and manage how network packets are forwarded between different networks and interfaces.

The routing table acts as a map that determines the path network packets take to reach their destination. By using the route command, administrators can control network traffic flow, troubleshoot connectivity issues, and optimize network performance.

While the route command is still widely used, it's considered a legacy tool from the net-tools package. Modern Linux distributions are gradually transitioning to the more powerful and versatile ip route command from the iproute2 package. However, many system administrators still prefer route for its simplicity and familiarity.

Installation

The route command is part of the net-tools package, which may not be installed by default on newer Linux distributions. To install it:

Debian/Ubuntu-based Systems

sudo apt update
sudo apt install net-tools

Red Hat/CentOS/Fedora Systems

# For CentOS/RHEL
sudo yum install net-tools
 
# For Fedora
sudo dnf install net-tools

Arch Linux

sudo pacman -S net-tools

Basic Syntax

The basic syntax of the route command is:

route [options] [command] [destination] [gateway] [netmask] [metric] [dev interface]

Where:

• options: Various flags that modify the behavior of the command
• command: The action to perform (add, del, etc.)
• destination: The destination network or host
• gateway: The gateway address for the route
• netmask: The network mask for the destination
• metric: The metric value for the route (priority)
• dev interface: The network interface to use

Basic Usage

Displaying the Routing Table

To display the current routing table:

route

This shows the routing table with hostnames and network names resolved.

Displaying the Routing Table in Numeric Format

To display the routing table with IP addresses instead of hostnames:

route -n

This is useful when DNS resolution is slow or unavailable, or when you need to see the actual IP addresses.

Understanding the Output

The output of the route command includes several columns:

• Destination: The destination network or host
• Gateway: The gateway address (an asterisk indicates no gateway is needed)
• Genmask: The netmask for the destination
• Flags: Various routing flags
  • U: Route is up
  • G: Route uses a gateway
  • H: Target is a host (not a network)
  • !: Reject route
• Metric: The distance to the target (lower is preferred)
• Ref: Number of references to this route
• Use: Count of lookups for the route
• Iface: The interface to which packets for this route will be sent

Advanced Usage

Adding a Default Gateway

To add a default gateway (the router that handles traffic for destinations not explicitly listed in the routing table):

sudo route add default gw 192.168.1.1

Replace 192.168.1.1 with your router's IP address.

Deleting the Default Gateway

To remove the default gateway:

sudo route del default

Caution: This may disconnect you from networks outside your local subnet.

Adding a Static Route

To add a static route to a specific network:

sudo route add -net 10.0.0.0 netmask 255.255.255.0 gw 192.168.1.254

This routes traffic for the 10.0.0.0/24 network through the gateway at 192.168.1.254.

Adding a Host Route

To add a route to a specific host:

sudo route add -host 10.0.0.1 gw 192.168.1.254

This routes traffic for the specific host 10.0.0.1 through the gateway at 192.168.1.254.

Specifying an Interface for a Route

To add a route that uses a specific network interface:

sudo route add -net 10.0.0.0 netmask 255.255.255.0 dev eth0

This routes traffic for the 10.0.0.0/24 network through the eth0 interface.

Rejecting Traffic to a Host or Network

To create a route that rejects traffic to a specific host:

sudo route add -host 192.168.1.51 reject

This causes the system to respond with "Network is unreachable" when attempting to communicate with 192.168.1.51.

Displaying the Kernel's Routing Cache

To display the kernel's routing cache (if available):

route -Cn

The routing cache helps speed up routing decisions by caching recent lookups.

Practical Examples

Setting Up Multiple Network Interfaces

If your system has multiple network interfaces connected to different networks:

# Configure the first interface
sudo route add -net 192.168.1.0 netmask 255.255.255.0 dev eth0
 
# Configure the second interface
sudo route add -net 10.0.0.0 netmask 255.255.255.0 dev eth1
 
# Set default gateway through eth0
sudo route add default gw 192.168.1.1 dev eth0

Creating a Static Route to a Remote Network

To access a remote network through a specific gateway:

sudo route add -net 172.16.0.0 netmask 255.255.0.0 gw 192.168.1.254

This allows your system to reach the 172.16.0.0/16 network through the gateway at 192.168.1.254.

Troubleshooting Network Connectivity

If you're having trouble reaching a specific network:

# Check the current routing table
route -n
 
# Add a specific route if needed
sudo route add -net 10.0.0.0 netmask 255.255.255.0 gw 192.168.1.254
 
# Test connectivity
ping 10.0.0.1

Setting Up a Router

To configure a Linux system as a router between two networks:

# Enable IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward
 
# Configure routes for each network
sudo route add -net 192.168.1.0 netmask 255.255.255.0 dev eth0
sudo route add -net 10.0.0.0 netmask 255.255.255.0 dev eth1

Making Routes Persistent

Routes added with the route command are not persistent and will be lost after a system reboot. To make routes persistent, you need to configure them in your distribution's network configuration files.

Debian/Ubuntu

Edit the interfaces file:

sudo nano /etc/network/interfaces

Add the following lines:

# For a static route
up route add -net 10.0.0.0 netmask 255.255.255.0 gw 192.168.1.254

# For a default gateway
up route add default gw 192.168.1.1

Red Hat/CentOS

Create a route configuration file for each interface:

sudo nano /etc/sysconfig/network-scripts/route-eth0

Add the following lines:

10.0.0.0/24 via 192.168.1.254 dev eth0

Using netplan (Ubuntu 18.04+)

Edit your netplan configuration:

sudo nano /etc/netplan/01-netcfg.yaml

Add routes to the configuration:

network:
  version: 2
  ethernets:
    eth0:
      routes:
        - to: 10.0.0.0/24
          via: 192.168.1.254

Apply the configuration:

sudo netplan apply

Comparison with ip route

The ip route command from the iproute2 package is the modern replacement for route. Here's a comparison of common commands:

Advantages of ip route over route

• More consistent syntax
• Support for advanced routing features (policy routing, multiple routing tables)
• Better support for IPv6
• More detailed output options
• Active development and maintenance

Common Options Reference

Troubleshooting

Common Issues

"SIOCADDRT: Network is unreachable"

This error occurs when you try to add a route through a gateway that's not directly reachable:

Solution: Ensure the gateway is on a directly connected network

"SIOCADDRT: File exists"

This error occurs when you try to add a route that already exists:

Solution: Delete the existing route first or modify it instead of adding a new one

Routes Not Working After Reboot

Routes added with the route command are not persistent:

Solution: Configure routes in your distribution's network configuration files

Unable to Reach Specific Networks

If you can't reach certain networks despite having routes configured:

Solution: Check for firewall rules that might be blocking traffic

Best Practices

Security Considerations

1. Avoid Unnecessary Routes: Only add routes that are necessary for your network topology
2. Use Reject Routes: Use reject routes to explicitly block traffic to sensitive networks
3. Consider Firewall Rules: Remember that routing and firewalling work together to control traffic flow

Performance Optimization

1. Minimize Route Lookups: Keep your routing table as simple as possible
2. Use Metrics Wisely: Set appropriate metrics to ensure traffic takes the optimal path
3. Consider Route Caching: On high-traffic systems, ensure the routing cache is properly sized

Documentation

1. Document Your Routes: Keep documentation of your routing configuration
2. Use Comments: Add comments in configuration files explaining the purpose of each route
3. Create Network Diagrams: Maintain network diagrams showing how routes connect different networks

Conclusion

The route command is a fundamental tool for managing network routing in Linux systems. While it's gradually being replaced by the more powerful ip route command, it remains widely used due to its simplicity and familiarity.

Understanding how to use route to view and manipulate the routing table is essential for network administrators, as it allows them to control how traffic flows through their networks, troubleshoot connectivity issues, and optimize network performance.

For modern Linux systems, it's recommended to learn both the traditional route command and its successor ip route, as you're likely to encounter both in various environments. The ip route command offers more features and better support for modern networking requirements, but the route command's straightforward syntax makes it valuable for quick checks and basic configuration tasks.